1. Field of the Invention
The present invention relates to an annular combustion chamber for a gas turbine engine such as a turbojet.
2. Description of the Related Art
FIG. 1 of the accompanying drawings is a longitudinal half-section showing a conventional combustion chamber 110. The other half of the chamber 110 can be derived by symmetry about the axis of the engine (not shown).
The combustion chamber 110 is located downstream from a diffusion chamber 130 constituted by an annular space defined between an outer casing 132 and an inner casing 134, into which diffusion chamber there is introduced an oxidizer, ambient air, that is compressed and that comes from an upstream compressor (not shown) via an annular diffusion duct 136.
In this specification, the terms “upstream” and “downstream” are used relative to the flow direction of gas through the engine.
The combustion chamber 110 has two concentric walls: a radially outer wall 112 (radial relative to the axis of the engine); and a radially inner wall 114; which walls are coaxial and substantially conical so as to provide the connection between the compressor flow section and the turbine flow section. The outer and inner walls 112 and 114 are connected together at the upstream end of the combustion chamber by a chamber end wall 116.
In this example, the chamber 110 is of the divergent type, i.e. the axis 200 of the combustion area diverges at an angle α relative to an axis 100 parallel to the axis of the engine. The outer and inner walls 112 and 114 of the combustion chamber 110 flare going from upstream to downstream.
The chamber end wall 116 is a frustoconical annular part that extends between two transverse planes, flaring from downstream to upstream. The chamber end wall 116 is connected to each of the outer and inner walls 112 and 114 of the combustion chamber 110 and it presents a shape that is slightly conical.
The chamber end wall 116 is provided with a plurality of openings that are angularly distributed around the axis of the engine, each of which receives a system 118 for injecting fuel pre-mixed with combustion air and through which there passes an injector 120 that introduces fuel into the upstream portion of the combustion chamber 110 where combustion reactions take place.
The effect of these combustion reactions is to cause heat to radiate from downstream to upstream towards the chamber end wall 116. Thus, in operation, the chamber end wall is subjected to high temperatures. In order to protect it, sectorized heat screens, referred to as deflectors 122, are interposed between the combustion area and the chamber end wall 116.
As shown in FIG. 2, each deflector 122 is generally in the form of a substantially plane plate made of refractory material and fastened to the chamber end wall 116 by brazing. It has two lateral margins forming rims 122b and 122c directed towards the chamber end wall 116, a radially outer edge 122f, and a radially inner edge 122e, together with a central opening 122a for passing the injector 120.
The central opening 122a is in alignment with one of the openings for receiving an injection system 118 in the chamber end wall 116. The radially inner and outer edges 122e and 122f of the deflector 122 form two guide nibs or tongues that are curved towards the combustion area and that leave a gap between the inner and outer walls 114 and 112 of the chamber 110.
The deflector 122 is cooled by the impact of jets of cooling air, represented by arrows in FIG. 3, which jets penetrate into the combustion chamber 110 through holes 124 formed in the chamber end wall 116.
The air constituting these jets, while flowing from downstream to upstream, is guided by chamber fairings 126, passes through the chamber end wall 116 via the cooling holes 124, and impacts against the upstream faces of the deflectors 122. The air is then guided radially towards the inside and the outside of the combustion area in order to initiate forming a film for cooling the inner and outer walls 114 and 112 of the chamber 110.
Guidance along the deflectors 122 is provided initially by the side rims 122b and 122c that extend radially. These rims 122b and 122c also perform a sealing function. Being in contact with, or leaving minimum clearance relative to, the end wall of the chamber 116, they prevent air from mixing between two adjacent deflectors 122, penetrating into the combustion area, and disturbing combustion.
Thereafter, the sheets of air for cooling the inner and outer walls 114 and 112 of the chamber 110 are initiated by the inner and outer guide nibs 122e and 122f of each deflector 122.
Unfortunately, it has been observed that hot points become distributed in regular manner around the circumference of the inner and/or outer walls 114 and/or 112 of the chamber 110, in particular in the radial planes containing the axes of the injectors 120.